?0 measurement with ALICE photon spectrometer PHOS in first proton-proton collisions at LHC

1
?0 measurement with ALICE
photon spectrometer PHOS in first
proton-proton collisions at LHC

• Lamia Benhabib
• SUBATECH (Nantes)

2
Overview

• Physics motivation to study the inclusive ?0
  spectrum in pp collisions
• Photon spectrometer PHOS
• Event rate estimation pT reached in first pp
  runs at LHC
• MC correction factor to obtain a realistic ?0
  spectrum
• ?0 geometrical acceptance
• ?0 efficiency reconstruction
• Influence of PHOS bad channels in ?0 efficiency
  reconstruction
• Conclusion

3
Physics motivation

• Test pQCD prediction
• Evaluate background for direct photon spectrum
• Reference measurement for heavy ion physics

4
From PHENIX results
From PHENIX, computing
vs N part Photon measurements show
almost no change from peripheral to central
collisions ?0 data show a loss and a
suppression of about factor 5 for more central
collisions
?0 measurement is a good probe for the parton
energy loss in the hot nuclear matter
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ALICE photon spectrometer PHOS
1 PHOS module

• 3584 crystals of PbWO4
• 1crystal dimension (1 cell ) 2.2x2.2x18 cm3
• Moliere radius 2cm
• Azimuthal angle 20
• Rapidity 0.12lt?lt0.12
• Granularity ??x??0.004x0.004
• Energy resolution 3.3/??E 1.1

• PHOS status/plan
• 1 PHOS module has been installed and will be
  operational for first proton-proton
• collisions at 10 TeV
• 3 modules are expected for the next LHC run

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Event rate estimation

• INCNLO program computes the photon and hadron
  differential cross sections
• p0 production is calculated in QCD hard
  processes at NLO g
  production at NLO involves 2 processes
• Hard direct photons gluon Compton scattering
• and q q annihilation
• Fragmentation

INCNLO P.Aurenche, M Fontannaz,
J.Ph.Guillet, B.A Kniehl, M. Werlen, Eur. Phys.
J.C13 (2000) 347.
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First proton-proton run condition
vs 10TeV
L 0.5 1029 cm-2s-1
T 30 days (effective run time is 30(1/10))
1 PHOS module Dh Df 0.24 20
1 PHOS module eg 0.9
 A?0 (pT)
Calculated with INCNLO program
A g,?0 e g Gustavo Conesa Thesis, Ref.
CERN-THESIS-2006-050
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Event rate at first proton-proton run condition
Production invariant cross sections for ?0 and
direct photons
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Event rate
Event rate for ?0 and direct photons
?0 can be detected up to 20 GeV/c and direct
photon up to 10 GeV/c for the first run at LHC
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How to obtain a realistic ?0 spectrum

• C MC which depends on
• C geom. accept. Geometrical acceptance
  correction
• C efficiency rec. Reconstruction efficiency
  correction
• L Measured luminosity
• T Run Time
• N ?0measured Number of measured ?0

C geom. Accept and C efficiency rec. have been
calculated from the following simulation
single ?0 events 0-25GeV 2000 events each bin
in ?255285 ?-0.150.15 (wider than one
PHOS module acceptance)
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Geometrical acceptance for ?0 ? ?
?0 with both decay ? in PHOS acceptance ?0
produced in y lt 0.5 and ?? 2 ?
C geom.accept.
Geometrical acceptance for ?0 is small for low
energy pions and saturates around 1 for E?0
gt25GeV.
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Reconstruction efficiency
reconstructed ?0 with 2 ? (or 3 ?)invariant mass
cut generated ?0 with 2 daughters in PHOS
C rec.eff

• Analysis method
• Fitting invariant mass spectrum
• Background subtraction
• Fitting invariant mass spectrum without
  background ? cut
• Calculate C rec

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Reconstruction efficiency Invariant mass
analysis
Invariant mass spectrum with background
Invariant mass spectrum without background
1920 GeV
E ?0 9,10GeV
E ?0 9,10GeV

• Background is fitted ploynomial function
  (poly2).
• A Gaussian fit for the invariant mass spectrum
  without background ? 4.7MeV

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Reconstruction efficiency with 2 ? and 3 ?
invariant mass cut
?
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Influence of bad channels in efficiency
reconstruction

• Reconstruction with a random bad map
  
  
• 70 Bad Channels (BC)    2 (a realistic bad
  channels map)
• 358 Bad Channels (BC) 10
• 896 Bad Channels (BC) 25
• Analysis method
• The same analysis method as before
  introducing a new parameter
  Distance To Bad Channel DBC which allows
  us to exclude a  bad channel  from the
  efficiency calculation.

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Influence of bad channels in efficiency
reconstruction (1)
Distance To BC 1 cell
Distance To BC 2 cells
1-?
1-?
Distance To BC 3 cells
1-?
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Influence of bad channels in efficiency
reconstruction (2)
10 of Bad Channels
2 of Bad Channels
1-?
1-?
25 of Bad Channels
1-?
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Conclusion

• We expect to measure ?0 up to 20GeV/c in the
  first proton-proton collisions at 10TeV with the
  ALICE photon spectrometer
• Geometrical acceptance and efficiency correction
  parameters have been estimated in order to obtain
  a realistic ?0 spectrum with PHOS
• ?0 efficiency correction parameter have been
  estimated as a function of bad channels and
  distance to bad channel
• Variation of acceptance and efficiency parameters
  will give the input for the systematic errors
  estimation